Just as the Warriors of Light had to defend their world from becoming engulfed by darkness, the Warriors of the Dark had to defend theirs from being swallowed by the light many years ago.
At the end of Final Fantasy III, the party has to find their dark counterparts so that they can aid them in their battle against the Cloud of Darkness. Each Warrior of the Dark appears after defeating each of the four bosses guarding the Dark Crystals in the World of Darkness: Cerberus, Echidna, Ahriman, and Two Headed Dragon.
After each battle, one of the warriors tells them of important information for the upcoming battle with the Cloud of Darkness. Before the final battle, the Warriors of the Dark sacrifice themselves so that the Light Warriors can save their own world.
At the beginning of their adventure, the Warriors of Darkness are chosen by the Crystal of Lux, and are blessed with its power. During their journey, they find five crystal shards which bless them with the Ranger, Dark Knight, Dancer, Ninja, and Magus Jobs.